ALBERT

All Library Books, journals and Electronic Records Telegrafenberg

X
feed icon rss

Your email was sent successfully. Check your inbox.

An error occurred while sending the email. Please try again.

Proceed reservation?

Export
  • 1
    Electronic Resource
    Electronic Resource
    Evaluation of MINDO/3 calculated structures. II. Branching errors in alkanes and cycloalkanesFor Part I see ref. 1. (1980)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 1 (1980), S. 233-239 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: MINDO/3 calculations have been carried out for a series of branched chain alkanes in order to assess effects of branching on calculated geometries and heats of formation (ΔHf). With vicinal branching, MINDO/3 calculates the central C—C bond to be too long. Bond angles are also found to be distorted. Errors in calculated heats of formation are large when geminal branching is present and significant with vicinal branching. Branching error corrections for ΔHf have been derived and applied to a separate series of branched acyclic and cyclic compounds. For the test sample, application of the branching error corrections gave calculated structures of acyclic branched hydrocarbons with heats of formation having an average absolute error of 1.3 kcal/mole rather than 17.3 kcal/mole before correction. Cyclic branched hydrocarbons are shown to be less well corrected. Calculations of heats of reaction have also been carried out for some isomerization and cyclization reactions using the MINDO/3 and MNDO methods. It is clear from the comparisons that MNDO calculations give less severe errors for highly branched compounds but the errors are still substantial. For prediction of heats of reaction, the error-corrected calculations are shown to be superior to the “raw” calculations obtained by MINDO/3 or MNDO.
    Additional Material: 7 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540010304
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 2
    Electronic Resource
    Electronic Resource
    Critical reevaluation of proximity effects in the barton oxidation and related intramolecular reactionsTaken from S. Vijayakumar, PhD Thesis, University of Toledo, Toledo, Oh, 1992. (1993)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 14 (1993), S. 1313-1319 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The limiting factor to account for the increased rate of intramolecular reactions between functional groups as opposed to their intermolecular counterparts can and has been explained both by proximity effects and by activation energy. Neither explanation has emerged as the single most important reason in all or even the majority of cases studied. We have therefore reexamined the spatiotemporal hypothesis of Menger and the transition-state energy approach of Houk on a consistent set of compounds subjected to the Barton oxidation or related reactions in an effort to more clearly define the reasons for the proximity effect. For the 26 structures studied, neither hypothesis provides a consistent, quantitative explanation although the transitionstate energy hypothesis offered the most promise. © John Wiley & Sons, Inc.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540141107
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 3
    Electronic Resource
    Electronic Resource
    Conformational distribution of heptaalanine: Analysis using a new Monte Carlo chain growth method (1992)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 13 (1992), S. 1216-1233 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A distribution of conformations of heptaalanine is obtained using a new Monte Carlo (MC) method that grows the chain atom by atom. Resulting configurations are energy minimized and a detailed analysis is performed of the minimum-energy conformers using a method of classification that partitions φψ space. The MC-generated configurations are compared with those generated from high-temperature molecular dynamics (MD) simulations. It is found that the new method generates a wide distribution of low-energy conformers at least 10 times more quickly than the MD. An analysis of the generated energy minima demonstrates that they can be divided into clusters in the space defined by the five pairs of φ - ψ angles of the inner residues. The space occupied by the minima populating each cluster is restricted. © 1992 by John Wiley & Sons, Inc.
    Additional Material: 10 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540131006
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 4
    Electronic Resource
    Electronic Resource
    Evaluation of MINDO/3 calculated structures. III. Saturated acyclic compounds with chlorine, nitrogen, oxygen, or sulfurFor Part II see ref. 1. (1982)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 3 (1982), S. 229-233 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Fifty-one structures have been calculated by the MINDO/3 method to evaluate the errors introduced by branching and by the presence of the heteroatom. The structures are evaluated by comparisons which reveal that the calculated ΔHf values reflect a bias because of the presence of the heteroatom. With two carbons or more in a chain attached to a heteroatom group, a linear relationship exists which makes possible the calculation of reasonably accurate ΔHf values for unbranched alcohols, primary or secondary amines, ethers, thioethers, thiols, and alkyl chlorides. Branching errors do not seem to be linearly related among the systems. Some errors in calculations of geometries are also discussed.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540030215
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 5
    Electronic Resource
    Electronic Resource
    An MNDO molecular orbital study of the reactions of protonated oxirane derivatives (XCHCH2OH+, X = CN, Cl, CH3, Ph) with simple nucleophiles. Implications for regioselectivity in the reactions of electrophiles with nucleic acid bases (1989)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 10 (1989), S. 568-592 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The gas-phase analogs of the classical SN1 and SN2 reactions of nucleophiles with a series of protonated oxiranes bearing the substituents CN, Cl, Me, and Ph were studied using MNDO semiempirical molecular orbital theory. The transition states for nucleophilic attack on the intact rings are calculated to become progressively more “SN1-like,” as the ability of the substituent group to stabilize the corresponding ring opened carbenium ion increases. At the same time the activation barriers become progressively smaller and their relative values for different nucleophiles approach the order expected on the basis of a purely electrostatic attraction between the reacting moieties. Exactly the opposite trends are calculated for the transition states for nucleophilic attack on the intermediate carbenium ions. As the stabilities of the latter increase the extent of bond formation, and transfer of charge to the incoming nucleophile also increase. Here, the relative barriers for attack by different nucleophiles approach the order expected on the basis of a superposition of both covalent and electrostatic interactions in which the former dominate. These results support a previously suggested rationalization for the enhanced reactivities of certain alkylating agents towards the exocyclic oxygen atoms, rather than the ring nitrogens, of the nucleic acid bases. They also suggest a new explanation for the tendencies of aralkylating electrophiles to modify the exocyclic amino groups of the nucleic acid bases: sites which are unreactive towards simple alkylating agents.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540100413
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 6
    Electronic Resource
    Electronic Resource
    Charge calculations in molecular mechanics IV: A general method for conjugated systems (1988)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 9 (1988), S. 288-297 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A general method for calculating pi charges based on the Hückel molecular orbital (HMO) approach is incorporated into an existing scheme for calculating partial atomic charges in saturated systems. The parametrization of the Hückel coulomb and resonance integrals is given based on (a) the CNDO pi densities and (b) the observed molecular dipole moments. The results for these two schemes are compared both with other theoretical methods and also with the observed dipole moments of a variety of conjugated and aromatic compounds. The dipole moment (DM) parametrization gives in general more consistent results and is to be preferred. The compounds studied include substituted olefins and aromatics (with substituents R, F, Cl, Br, I, OR), aldehydes, ketones, acids, esters, amides, and the heterocyclic ring systems of furan, pyrrole, pyridine, and indole.
    Additional Material: 6 Tab.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540090403
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 7
    Electronic Resource
    Electronic Resource
    Comparison of the molecular mechanics + generalized Born/surface area and the ab initio + Monte Carlo simulation methods in estimating conformational equilibria in aqueous solution (1994)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 15 (1994), S. 1228-1240 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: A method using molecular mechanics to calculate internal energies and the generalized Born/surface area (GB/SA) method for calculating solvation free energy has been compared with the corresponding terms obtained from ab initio quantum chemical calculations in the gas phase and the free energy perturbation method implemented in Monte Carlo (MC) simulations to study conformational equilibria in solution. 1,2-Ethanediol, 2-hydroxybenzoic acid, the neutral and protonated histamine, were considered in aqueous solution as systems capable of intramolecular hydrogen bonding. The molecular mechanics method with all atom and united atom models, using the AMBER* force field and the parameterization as implemented in the MacroModel modeling package and the GB/SA continuum solvation model, produces smaller separation in relative conformer free energies than does the ab initio + MC method in aqueous solution. The GB/SA relative solvation free energies using the charges from the AMBER* force fields were consistently smaller than the values obtained in Monte Carlo simulations. Using the charge sets from the Monte Carlo simulations and considering solute geometries with torsional angles fixed at the optimized ab initio values, the relative solvation free energies remained underestimated by up to 30% as compared to the Monte Carlo values. The AMBER*//GB/SA predicted most stable conformer for the 1,2-ethandiol system in aqueous solution is in contrast with the ab initio + MC finding and the available experimental results. For the histamine system predictions by only the united atom AMBER*//GB/SA model agree with those by ab initio + MC and with data derived from nuclear magnetic resonance (NMR) experiments. Differences in the two methods are considered mainly due to the application of torsional parameters and atomic charges developed in the AMBER* parameter set for monofunctional polar systems. New parameters seem to be needed for quantitative description of the in-solution conformational equilibria for organic compounds with a possible intramolecular hydrogen bond. © 1994 by John Wiley & Sons, Inc.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540151105
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 8
    Electronic Resource
    Electronic Resource
    Derivation of net atomic charges from molecular electrostatic potentials (1990)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 11 (1990), S. 297-310 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The dependence of net atomic charges, as derived from least-squares fitting to electrostatic potentials, on molecular orientation and potential site location is critically examined. A unique rotationally invariant algorithm, in which the charges may be constrained to reproduce the molecular dipole moment, is presented and the results for several basis sets compared to previous work. Significant improvements in dipole moments derived from the unconstrained charges are noted.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540110304
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 9
    Electronic Resource
    Electronic Resource
    Conformational preferences of XONO2 systems (X = H, F, Cl, CH3) from ab initio techniques (1991)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 12 (1991), S. 565-574 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Ab initio SCF and MP2 methods have been used to calculate the geometries and relative energies of both planar and nonplanar rotamers of several nitrates with a DZP basis set. The planar arrangement of atoms is found to be the lower energy configuration in all cases. The interconversion between rotamers is analyzed by partitioning the contributions to the total SCF energy in a variety of ways.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540120506
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
  • 10
    Electronic Resource
    Electronic Resource
    Numerical sensitivity of trajectories across conformational energy hypersurfaces from geometry optimized molecular orbital calculations: AM1, MNDO, and MINDO/3 (1988)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 9 (1988), S. 387-398 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: The monocyclic β-lactam [[4(S)-methyl-2-oxo-1-azetidinyl]thia]acetic acid was studied by the semiempirical molecular orbital methods AM1, MNDO, and MINDO/3. Using the reaction coordinate option in the program MOPAC on VAX and Cray X-MP computers, the potential energy curve was calculated for rotation of the C2-N1-S-C torsional angle in the conformationally flexible side chain while optimizing all other geometrical variables in the molecule. The trajectory taken during geometry optimization was found to be sensitive to the computer, the program version, the convergence criteria, and the degree of code optimization used in the calculation. In order to reduce the likelihood of spurious results, conformational or reaction energy hypersurfaces need to be calculated with the more precise SCF convergence and minimization criteria available in programs for MINDO/3, MNDO, and AM1 calculations. The nitrogen in the model β-lactam antibiotic is predicted to invert periodically as the dihedral angle to the exocyclic N-substituent sweeps through 360°.
    Additional Material: 6 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540090414
    Location Call Number Expected Availability
    BibTip Others were also interested in ...
    Paper (German National Licenses)
    Fulltext
Close ⊗
This website uses cookies and the analysis tool Matomo. More information can be found here...